Compositional control method



United States Patent O M orsto the United States of America as represented by the Secretary of the Navy 7 H .No Drawing. .Filed Jan. 25, 1956, Ser.No. 561,371

Claims. (Cl-23-230) {Granted under Title 35, US. Code (1952), sec. 266) The invention described herein-may .be manufactured I i and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates generally to methods of maintaining control of chemical compositions and more particularly, to a test procedure for use in the compositional control ofp'repaint rust inhibiting .solutions.

In the painting of steel and othermetallic structures which are subject to rust, ,a common procedure is to submit the structure to a pickling step and then to immerse the structure in a hot, rust-inhibiting solution. Pickling is employed to remove mill scale and rust from steel plates, :shapes or structures and consists ofimmersion in a hot, dilute acidsolution such as sulphuric acid. The

excess acid is usually removed by a hot water rinse. The

pickled structure is then immersed in a hot rust-inhibiting .solution which may usually consist "of several pounds, say,

about 6 to 7 pounds of sodium, dichromate and about 5 to 6-pounds of phosphoric acid (75%) per 100 gals. of solution. For effective operation, it is generally preferred that the strength of the rust-inhibiting solution be main- .tained at 50% to 100% of the aforementioned strength.

The maintenance of rust-inhibiting solutions at proper strength necessitates vigilant control and frequent analyses.

. It'has been found that the rust-inhibiting solution may changeconsiderably in both sodium dichromate and phosphoric acid content during a single busy workday. Ac-

' cordingly, on the spot, daily compositional control of the solution should be maintained. Such control may be exercised by the use of well-known titrimetric methods both for the determination of the concentrations of the phosphoric acid and the sodium dichromate. Such titrimetric methods are accurate but usually require special- ,ized equipment such'as a pH meter and also require a I .high degree of .skill in the operating personnel.

It is, accordingly, the primary objectof the present invention to provide a simple method for on-the-spot control of rust-inhibiting solutions which is satisfactorily accurate, requires little or no special equipment, and which may be carried out by relatively unskilled operating personnel.

7 It is a further object to provide a method as set forth in the preceding object which employs color comparison to determine the concentration of constituents in the rustinhibiting solutions.

It is another object to provide a color-comparison method for on-the-spot control of rust-inhibiting solutions .in which it rnay readily be determined whether or not the inhibiting solutions have been depleted to the half-strength point without requiring a reference to a standard color solution.

In accordance with the present invention, there is provided a method of determining. the concentration of a dichron atesalt and phosphoric acid in a solution comprising aliquoting first and second prescribed portions of said solution into separate vessels,,adding a small amount of methyl purple indicator to said first portion to indicate 2,931,930 Patented .May .24, 1960 2 the degree of acidity thereof whereby a relationship between said degree of acidity and the concentration of said phosphoric acid, the color of said solution being purple below a pH of 4.8, green above a pH of 'SAjand light gray to colorless at a pH of 5.1, providing a small prescribed portion of a coloring agent solution containing cobalt sulfate, copper sulfate, and phosphoric acid to said second portion with the resulting solution containing said coloring agent being colorless at a given concentration of said dichromate, purple below said concentration, and yellow above said concentration.

Other objects and many of the attendant advantages of this invention will be readily appreciated as 'thesamejbecomes better understood by reference "to the following detailed description. 7

To carry out the method of the present invention, standard solutions of phosphoric acid and sodium dichromate solutions are first prepared.

PREPARATION OF PHOSPHORI C ACID STANDARDS A group of square Pyrex bottles, say, about -6, are thoroughly cleaned with a suitable cleaning material such as sulfuric acid-dichromate cleaning solution. cleaned bottles are then rinsed well with distilledwater and soaked in distilled water for about 72 hours. The bottles may be conveniently numbered 0, 1, 2, 3, 4, and 5. The phosphoric acid solutions are prepared asfollows:

(l) A quantity of 10.0 ml. of phosphoric acid is diluted to 1 liter.

(2) To 62.1 ml. of solution (1), there is added. 1.35 grams of sodium dichromate Na CrgOq-2H 0 and the solution is diluted to 1 liter with distilled water.

(3) To 5 ml. of solution (2), there are added 745 ml. of distilled water and the solution is well mixed.

4 Standard solutions equivalent to 0, 1, 2, 3, '21, an 5 pounds of phosphoric acid (75 percent) per gallons of inhibiting solution are prepared by using 50ml. of solution (3) for each pound of phosphoric acid (75 i percent per 100 gallons of mhibitmg solution and dilutingtoSO?) mlzswith distilled water. a

) The bottles cleaned and numbered i set forth above, are filled with the corresponding solutions of phosphone acid concentrations and 3 drops of methylpurple indicator are added to each bottle, the bottles are tightly capped and the contents therein are mixed byinverting several times. The bottles marked 0, 1, and 2 will be I green with the intensity decreasing as the concentration of phosphoric acid increases, and those marked 3, 4, and 5 will be purple with the intensity increasing as the con centration of phosphoric acid increases. 7 are preferably stored in a cool dark place.

(6) The distilled water used in the preparation of these standards as well as that used in the 'dilutionof the samples preferably should not be exposed to the atmosphere longer than absolutely necessary to avoid absorption of carbon dioxide. In this connection, polyethylene bottles are preferably used in the place of glass bottles in the event that it is desired to store distilled water.

Methyl-purple indicator is a material manufactured by the Fleisher Chemical Company of Washington, D'. C., and is disclosed in United States Patent No. 2,416,619; It is a mixed indicator consisting of methyl-red (sodium salt) plus a blue dye. A dilute solution of methyl redl is red below a pH of 4.2 and yellow above a pH of 6.31 The color of the blue dye is not affected by changes in pH. The resultant combined-indicator color is purple below a pH of 4.8, green above a pH of 5.4, and light gray to colorless at a pH of 5.1. Intensity of the purple or green color varies with pH within definite The.

The standards PREPARATION OF SODIUM DICHROMATE STANDARDS (1) Eight matched Nessler tubes are marked with the numbers'O, 1, 2, 3', 4, 5, 6, and 7. 4

- (2) A quantity of 1.197 grams of sodium dichromate (Na Cr O -2H O) is dissolved in distilled water and the solution is diluted to 1 liter.

(3) A volume of 50.0 ml. of solution (2.) is diluted to 1 liter with distilled water.

(4) Standard solutions equivalent to O, l, 2, 3, 4, 5, 6, and 7 pounds of sodium dichromate per 100 gallons of rustinhibiting solution are prepared by adding 5.0 ml. of solution (3) for each pound of sodium dichromate to the correspondingly numbered aforementioned Nessler tubes. Each of the solutions are diluted to 100 ml. with distilled water. To each diluted solution, there is now added 5.0 ml. of the color reagent for sodium dichromate. The Nessler tubes are capped and the contents thereof are mixed by inverting several times. It is preferable that these standards not be exposed to sunlight and be stored in a dark place.

The color reagent for the sodium dichromate determin-ation is prepared by dissolving 50 grams of cobalt sulfate, CoSO -7H O, 150 grams of copper sulfate, CuSO -5H O, and 50 ml. of phosphoric acid (85%) in distilled water and diluting the solution to 1 liter. As is seen, this color reagent consists of solutions of blue copper sulfate and red cobalt sulfate to produce a purple color. Phosphoric acid is added to prevent hydrolysis of the salts and provide the hydrogen ion concentration necessary to maintain the desired dichromate color. A mixture of inorganic salts is used because the colors are stable and not afiected by dichromate oxidation. When the dichromate standards are examined in the white light of an illuminator such as a 15 watt white fluorescent tube, the following colors are observed.

(a) The 0, l, and 2 standards are purple, with the intensity decreasing as the concentration of sodium dichromate increases.

(b) The number 3 standards, corresponding approximately to one-half the nominal strength of 6.3 pounds of sodium dichromate per 100 gallons of rust-inhibiting solution, is practically colorless.

(c) The 4, 5, 6, and 7 standards are yellow, with the intensity increasing as the concentration of sodium dichromate increases.

In carrying out the method of the present invention, about 1 to 2 ounces of filtered rust-inhibiting solution being monitored is cooled to roomtemperature. A quantity of 0.5 ml. of the filtered solution is diluted to 200 ml. and well mixed. An amount of 21.4 ml. of the diluted solution is poured into a square Pyrex glass bottle similar to those used in preparing the phosphoric acid color standards and water is aded until the volume reaches 160 ml. To this 160 ml. solution is added exactly 3 drops of the aforedescribed methyl purple indicator. The color of the solution is visually compared with the standard, preferably using a white background to determine the number of pounds of phosphoric acid (75 percent) per gallon of inhibiting solution. if the color of the solution containing the sample does not exactly match any of the standard solutions, fractional estimates may be made.

The concentrations of phosphoric acid in the final solutions containing the sample of the rust-inhibiting solution is such that at one-half the nominal strength of the rustinhibiting solution, say about 2.8 pounds of phosphoric acid (75%) per 100 gallons of solution, the test solution containing the methyl-purple is colorless, purple above this half-strength concentration point and green below this concentration point. Above 4.5 pounds per 100 gallons, the pH is outside the range of sensitivity of the indicator, and further dilution of the sample is necessary.

To determinesodium dichromate concentration, a filtered quantity 0.25 ml. of rustdnhibiting solution is diluted to ml. and 5 ml. of the color reagent for the sodium dichromate is added thereto. The color of the resulting solution is compared visually against the sodium dichromate standard solutions, preferably utilizing an illuminator and looking down through the full depth of 'the solution, to determine the number of pounds of sodium dichromate per 100 gallons of rust-inhibiting solution. When the sample does not exactly match any of the standard solutions, fractional estimates may be made. In a half-strength inhibiting solution, say having a concentration of 3.15 pounds of sodium dichromate per 100 gallons of solution, the color reagent exactly blanks out the yellow color of sodium dichromate solution, preferably as observed in white light to produce a virtually colorless or grey solution. The solution is purple below half strength, and yellow above. Intensity of color obviously varies with sodium dichromate concentration.

In the following table, there is set forth results obtained using the color comparison method of the. present invention as compared with exact chemistry laboratory quantitative titrirnetric determination.

Table of results [Pounds per 100 gallons of inhibiting solution] Phosphoric Acid (75%) Sodium Dichromate Color Quanti- Quanti- Comparitative Difi. Color Oomtative Difi.

son Lab. parison Lab.

Analysis Analysis An examination of the results shown in the table demonstrates that the deviation of the color comparison method from more exact laboratory quantitative determination is in each case less than one-half pound of ingredient per 100 gallons. The color comparison test is made by relatively unskilled personnel, requires approximately 15 minutes, and is substantially accurate for routine control purposes.

It is also seen that if the half-strength depletion point of the rust-inhibiting solution is the point at which it should be replenished, making the concentration of the constituents in the tested sample such that at the hall? strength depletion point, there is no color in the phosphoric acid and sodium dichromate tests. Therefore, a method is provided wherein the operating personnel can immediately determine whether the concentrations of the phosphoric acid and the sodium dichromate in the rustmhibiting solution are above or below the half-strength depletion point without necessitating any color comparison standards. 1

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim: I

l. A method of determining the concentration of .dichromate salt in a test solution in combinationwiththe utilization of comparison standards comprising a plurality of comparison solutions of different but progressively varying concentrations of dichromate saltsand each of said comparison solutions having incorporated therein a prescribed but equal quantity of a color indicator; said indicator being an aqueous solution containing CuSO .5H O and 50 ml. of phosphoric acid (85%) per liter of solution.

3. A method of determining the unknown concentration'of a'dichromate salt in a solution with respect to a given concentration value of said salt comprising adding a small amount of a color indicator to a chosen specimen of said solution, said color indicator comprising a solution containing CoSO .7H O, CuSO .5H O and phosphoric acid, .the color of said resulting solution being substantially colorless at said given concentration, yellow above said given concentration and purple below said given concentration and both colors increasing progressively in color intensity with changes in concentrations in both directions away from said given concentrations,

whereby a comparison of the resultant color of said specimen with added indicator with solutions of known but progressively varying concentrations of said salt having equivalent amounts of said indicator will indicate the approximately concentration of said salt in said solution of unknown concentration.

4. The method as defined in claim 3 wherein said color indicator comprises an aqueous solution containing about 50 grams of CoSO .7H- O, 150 grams of CuSO .5H O and 50 m1. of phosphoric acid (85%) per liter of solution.

5. A method of determining during use whether or not a solution in use of a rust inhibiting sodium dichromate solution has an adequate minimum concentration strength to perform its intended function of rust inhibition, which comprises preparing a plurality of separate comparison solutions of sodium dichromate of progressively graduated strengths, adding to each of said solutions a color indicator agent for sodium dichromate that does not chemically react materially with the sodium dichromate in equal amounts adequate to produce a sequence of colors varying progressively in intensity from an intermediate strength solution that is practically colorless and is a minimum safe concentration of the dichromate to purple colors of progressively increasing intensities as the concentrations of the sodium dichromate decrease progressively, and to a yellow color of progressively increasing intensities as the concentration of the sodium dichromate progressively increases, also at intervals during use of the said solution in use, removing a filtered specimen thereof and adding to it a measured amount of said color agent, equivalent to that added to each of such comparison solutions, and placing said filtered specimen with color agent in a position for visual comparison of its color with the said comparison solutions of progressively varying strengths whereby a visual observation of References Cited in the file of this patent UNITED STATES PATENTS 1,462,840 Wilkinson July 24, 1923 2,416,619 Fleisher "Feb. 25, 19 47 7 2,569,663 'Frank et al. Oct. 2, 195l OTHER REFERENCES Snell: Colorimetric Methods of Analysis, vol. I, 3rd ed., 1948, pages 62.

Diamond Chromium Chemicals-General Use and Applications, publication of the Diamond Alkali Co., Cleveland 14, Ohio, pages 23-25. 

1. A METHOD OF DETERMINING THE CONCENTRATION OF DICHROMATE SALT IN A TEST SOLUTION IN COMBINATION WITH THE UTILIZATION OF COMPARISON STANDARDS COMPRISING A PLURALITY OF COMPARISON SOLUTIONS OF DIFFERENT BUT PROGRESSIVELY VARYING CONCENTRATIONS OF DICHROMATE SALTS AND EACH OF SAID COMPARISON SOLUTIONS HAVING INCORPORATED THEREIN A PRESCRIBED BUT EQUAL QUANTITY OF A COLOR INDICATOR, SAID INDICATOR BEING AN AQUEOUS SOLUTION CONTAINING 